Plasmonic distributed feedback lasers at telecommunications ... - Tue

Plasmonic distributed feedback lasers at telecommunications wavelengths Milan J.H. Marell,1 Barry Smalbrugge,1 Erik Jan Geluk,1 Peter J. van Veldhoven,2 Beatrix Barcones,2 Bert Koopmans,2 Richard Nötzel,2 Meint K. Smit,1 and Martin T. Hill1,* 1

Dept. Electrical Engineering, Technische Universiteit Eindhoven, Postbus 513, 5600 MB, Eindhoven, The Netherlands 2 Dept. Applied Physics, Technische Universiteit Eindhoven, Postbus 513, 5600 MB, Eindhoven, The Netherlands *[email protected]

Abstract: We investigate electrically pumped, distributed feedback (DFB) lasers, based on gap-plasmon mode metallic waveguides. The waveguides have nano-scale widths below the diffraction limit and incorporate vertical groove Bragg gratings. These metallic Bragg gratings provide a broad bandwidth stop band (~500nm) with grating coupling coefficients of over 5000/cm. A strong suppression of spontaneous emission occurs in these Bragg grating cavities, over the stop band frequencies. This strong suppression manifests itself in our experimental results as a near absence of spontaneous emission and significantly reduced lasing thresholds when compared to similar length Fabry-Pérot waveguide cavities. Furthermore, the reduced threshold pumping requirements permits us to show strong line narrowing and super linear light current curves for these plasmon mode devices even at room temperature. © 2011 Optical Society of America OCIS codes: (250.5403) Plasmonics; (250.5960) Semiconductor lasers.

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Received 28 Apr 2011; revised 12 Jun 2011; accepted 14 Jun 2011; published 21 Jul 2011

1 August 2011 / Vol. 19, No. 16 / OPTICS EXPRESS 15109

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1. Introduction Lasers based on metallic cavities have opened up a new route for laser miniaturization in the last few years [1–7]. The myriad of concepts presented include propagating plasmon mode devices, an example of the SPASER concept [8] which is deep-subwavelength in all three dimensions, and metal encapsulated devices which tightly confine the light to close to the diffraction limit or even below in some dimensions. Most devices are optically pumped and use dye or semiconductor gain medium, however some semiconductor devices are electrically pumped, which brings them closer to applications. Furthermore, there has also been work looking at the theory behind such devices [9,10], showing that some designs have the potential for useful devices. One of the most promising approaches for both the construction of lasers, active and passive components with true sub-wavelength size is the use of metal-insulator-metal (MIM) waveguide structures which guide gap-plasmon modes [11]. Lasing in Fabry-PérotPérot (FP) cavities made from sections of such metallic waveguides has already been shown [12]. However, for many applications the precise control of the lasing wavelength and the optical mode in a waveguide laser cavity is highly desirable. Additionally, for optical signal processing applications and also out-coupling of laser light to passive waveguides, some form of mirror with controlled reflectivity and frequency selection is required. Incorporation of Bragg gratings into the waveguide to form distributed feedback (DFB) or distributed Bragg reflectors (DBR) [13,14] is common in dielectric devices and also plasmonic structures operating in the far-infrared [15,16]. To implement Bragg type reflectors in an MIM waveguide structure, the width of the MIM waveguide can be modulated to form vertical groove gratings [17] as shown in Fig. 1. Broad high reflectivity stop bands are possible in such structures (Fig. 2). The optical intensity plots (obtained from finite difference time domain FDTD simulations) for the resonant modes that occur in such waveguide grating structures are given in Fig. 3.

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Our version of the MIM waveguide includes a thin insulator region between the semiconductor and metal (metal insulator semiconductor insulator metal - MISIM waveguide), and contains a higher index gain region (InGaAs) to localize the propagating mode [12,18]. Additionally the InGaAs gain region is pumped via an electrical n-contact located on the top of the ridge and p-contact via the substrate [12] (Fig. 1). We have chosen a simple InGaAs gain medium due to our experience in growing it. A quantum well gain medium may be a better alternative [6].

Fig. 1. (a) Schematic overview of the device. The semiconductor core consists of an InP/InGaAs/InP heterojunction. The core is shielded from the silver cladding by a SiNx layer. (b) Shows a top view of a single grating section.The characteristic parameters of the structure are wc, wg and Lp, which represent the width of the waveguide core, the width of a grating section and the period of the grating. tm and td are the thicknesses of the metal cladding and dielectric insulation layer respectively. A quarter wavelength phase shift is placed in the center of the grating.

Fig. 2. Typical reflection spectrum from a three period vertical groove grating, based on the MI-S-I-M waveguide structure, wc = 100 nm + x nm, red x = 80, magenta x = 60, green x = 40, blue x = 20, td = 20 nm .

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Fig. 3. (a) Simulated electric field intensity (|E|2) plot of a slice through the waveguide (z = 0 plane). The plot shows the existence of a well confined cavity mode centred on the InGaAs layer. Furthermore, that the field is strongest in the SiNx region between the semiconductor and the metal. (b) Field distribution of the mode at the Bragg wavelength with the mode centered on the quarter wavelength phase shift, and also at the first band-edge mode, where the mode is spread out through the grating. Plots show slices through the z = 0 plane.

2. Device fabrication The InP/InGaAsP/InGaAs layer stack [1] as shown in Fig. 1 and Fig. 3a was grown via metal organic vapour phase epitaxy on a double polished semi-insulating InP substrate. All InGaAsP/InGaAs layers were lattice matched to InP. Electron beam lithography and reactive ion etching (RIE) was used to form a SiO2 hard mask to define the ridge/DFB cross sections. The ridge was formed by inductively coupled plasma (ICP) RIE, then a number of surface oxidation and wet chemical oxide removal steps, to remove surface damage and modify the sidewall profile. The silicon nitride layer was deposited using plasma enhanced chemical vapor deposition. Planarization with resist was used to expose just the tops of the pillars, to form the top ncontact. Various metal layers were deposited mostly via electron beam evaporation. However, the silver encapsulation was deposited via thermal evaporation and annealed at 400°C for 60 seconds. A lateral p-contact was formed via the p-InGaAsP layer. The processed wafer was cleaved into sections containing eight devices. The sections were mounted with the substrate facing up on carriers, and wire bonds made to individual devices. For selected devices focused ion beam milling was used to open the end facets of the MIM waveguides, to allow direct measurement of light travelling along the waveguides. Figure 4 shows scanning electron microscope (SEM) pictures of a section of the semiconductor waveguide core before encapsulation in metal (silver), and a cross-section of one of the waveguides.

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Fig. 4. (a) Scanning electron microscope (SEM) photo of the DFB structure without the surrounding silver and nitride layer. All scale bars represents 100 nm. (b) A cross-section of DFB structure, with surrounding silver and gold layers. The dark line between the metal cladding and the semiconductor core is the nitride insulation layer. The width of the semiconductor core is approximately 100 nm.

3. Waveguide modes For the gap-plasmon (Transverse Magnetic, TM0) modes that propagate in such waveguides, a significant proportion of the modal energy is carried in the thin insulator region between the metal and semiconductor (Fig. 3a). Small changes in the width of the waveguide core can result in a large modal mismatch and a strong reflection, which can be used to achieve Bragg grating coupling coefficients 10 to 100 times larger than in dielectric gratings. From FDTD simulations of a section of grating length L, the reflectivity R can be found, and from this the grating coupling coefficient κ = tanh1(R1/2)/L can be shown to reach 5000 cm1 over a bandwidth of 500 nm, centered at 1550 nm. The M-I-S-I-M waveguide parameters for the above grating were (Fig. 1b): wc = 100 nm, wg = wc + 50 nm, Lp = 230 nm, td = 20 nm. A cavity made from two such gratings with a quarter wavelength shift between them should have a quality factor (Q) of ~300 at room temperature. This Q is primarily determined by the optical losses in the metal [19]. The optical gain from the InGaAs required to overcome losses in the cavity can be found [20] to be approximately 1200 cm1, a value which can be obtained from bulk InGaAs material. (Fabry-Perot devices have a similar Q and modal confinement, and hence a similar threshold gain). The strong grating coupling means only a few periods (